blob: Slightly enforce a create_fromchunks() test
odb: Prevent stream_finalize_write() from overwriting

Now that #1785 is merged, git_odb_stream_finalize_write() calculates the object id before invoking the odb backend.

This commit gives a chance to the backend to check if it already knows this object.
Merge pull request #1815 from libgit2/ntk/topic/stream_write/check_before_overwriting

Ask the odbbackend if the object exists before overwriting it